Abstract:

A two-stage treatment process, for the treatment of nitrate rich water,
particularly aquaculture pond water, wherein in the first stage a
degassing chamber is used for removing dissolved oxygen from a stream of
water flowing out of the aquaculture system, and in the second stage the
stream of water obtained from said degassing chamber is flown into a
denitrifying biofilter comprising a biofilter media which functions as a
biological growth media and as a carbon source, wherein said denitrifying
biofilter is capable of biologically reducing both nitrate and nitrite
compounds into nitrogen gas.

Claims:

1. A denitrification apparatus comprising a degassing chamber adapted to
remove dissolved oxygen from a stream of water flown thereinto, and
anoxic biofiltering means capable of carrying out denitrification of a
stream of water received from said degassing chamber.

2. The denitrification apparatus according to claim 1, wherein the
degassing chamber also removes carbon dioxide.

3. The denitrification apparatus according to claim 1, wherein the
degassing chamber comprises a water tank having a water inlet provided in
an upper portion thereof and a water outlet provided in a lower portion
thereof, and wherein said water inlet is connected to a spray nozzle
installed in said tank, and wherein a vacuum pump connected to an upper
portion of said tank, is used for applying negative pressure there
inside.

4. The denitrification apparatus according to claim 1, wherein the anoxic
biofiltering means comprises an elongated vessel comprising a water inlet
and a water outlet provided in opposing sides thereof, such that water
streamed therethrough is flown along a length of said vessel, and one or
more biofilter medias disposed thereinside.

5. The denitrification apparatus according to claim 4, wherein, the one or
more biofilter medias are disposed along the length of the elongated
vessel covering cross-sectional sections thereof such, that water flown
thereinside is forced to pass through said one or more biofilter medias.

6. The denitrification apparatus according to claim 5, further comprising
a plurality of spacer elements filling sections of the elongated vessel.

7. The denitrification apparatus according to claim 6, wherein the one or
more biofilter medias comprise materials capable of functioning as growth
media and as a Carbon source.

8. The denitrification apparatus according to claim 7, wherein the one or
more biofilter medias comprise cotton-wool.

9. The denitrification apparatus according to claim 6, wherein the spacer
elements are small porous balls or beads.

10. The denitrification apparatus according to claim 1, further comprising
a water pump for supplying the stream of water to the degassing chamber.

11. A method for denitrifying water, comprising: providing a stream of
water, removing dissolved oxygen from said stream of water and thereafter
filtering said stream of water by means of one or more biofilter medias
capable of functioning as growth media and as a Carbon source for
denitrifying bacteria.

12. The method according to claim 11, wherein the filtering is carried out
in an elongated vessel having one or more biofilter medias installed
along its length, and wherein the stream of water is flown along the
length of said elongated vessel.

13. The method according to claim 12, wherein a uniform water stream is
obtained in the elongated vessel by means of a plurality of spacer
elements filling portions of said elongated vessel.

14. The method according to claim 13, wherein the plurality of spacer
elements minimize pressure drops and prevent compaction and clogging of
the biofilter media.

15. A water treatment system comprising: a source of water, a degassing
chamber adapted to receive a stream of water from said water source and
remove dissolved oxygen therefrom, an anoxic biofiltering means adapted
to denitrify a stream of water received from said degassing chamber by
means of a biofilter media capable of functioning as a biological growth
media and as a carbon source, an aerobic biofiltering means adapted to
nitrify water streams received from said water source and from said
anoxic biofilter and provide a nitrified stream to a water filtering
means connected thereto.

16. The water treatment system according to claim 15, wherein the water
filtering means is a type of particle sand filter.

17. The water treatment system according to claims 15, wherein the anoxic
biofiltering means comprises an elongated vessel having one or more of
the biofilter media disposed along its length and a plurality of spacers
filling sections of said elongated vessel.

Description:

FIELD OF THE INVENTION

[0001]The present invention generally relates to an apparatus and method
for treating water, and in particular, to an apparatus and method for the
denitrification of wastewater.

BACKGROUND OF THE INVENTION

[0002]The present invention aims to provide a compact water treatment
system for the removal of nitrate (and nitrite), which is particularly
useful for aquaculture (aquafarming) systems including, but not limited
to, small, remote and urban systems.

[0003]Maintaining acceptable water quality is without doubt the bottleneck
in recirculating aquaculture systems (vanRijn, J. The potential for
integrated biological treatment systems in urecirculating fish culture--A
review, Aquaculture, 1996, Vol. 139, pp. 181-201). The most common
problems in such systems are the accumulation of inorganic Nitrogen
compounds, particularly Ammonia, Nitrite and Nitrate. In order to curb
these effects, biological treatment systems are usually used.
Nitrification, where Ammonia is oxidized to Nitrate through Nitrite is
known by:

55NH4++76O2+109HCO3-C5H7O2N+54NO.s-
ub.2-+57H2O+104H2CO3 1.

400NO2-+NH4++4H2CO3+HCO3-+195O.sub-
.2C5H7O2N+3H2O+400NO3- 2.

[0004]A popular and economically feasible method to remove
ammonia/ammonium, by nitrification, is through the use of trickling
filters. The importance of bed substrate in nitrifying biofilters is
immense (Kim, S. K. et al., Removal of ammonium-N from a recirculating
aquaculture system using an immobilized nitrifier, Aquacultural
Engineering, 2000, Vol. 21, pp. 139-150). If efficient nitrification is
to take place, the bed substrate needs to be porous, durable, and low in
cost, have a high surface area to volume ratio, not to clog easily, and
to supports a homogenous flow of water.

[0005]Denitrification, the process wherein Nitrate is reduced to Nitrogen
gas (also through nitrite), is defined by:

1000NO3-+880CH3COO-+H+88C5H7O2N+46-
0N2+420CO2+880HCO3-+1070H2O 3.

[0006]The process of biological denitrification is carried out by
facultative anaerobic bacteria, which in the presence of a carbon source,
and in the absence of dissolved gaseous oxygen carry out the process.
Furthermore, denitrification serves to increase the buffering capacity of
the system (vanRijn, 1996). Additionally, providing an anaerobic
environment not only serves to remove Nitrate, but can also reduce the
total system phosphate concentrations (Barak, Y., vanRijn, J., Biological
phosphate removal in a prototype recirculating aquaculture treatment
system, Aquacultural Engineering, 2000, Vol. 22, pp. 121-136), and be
applied for the removal of various contaminants present in water and
wastewater.

[0007]Environmentally friendly recirculation systems, which conform to
strict environmental legislation, are acknowledged as a needed, feasible
approach, both technically and economically, for inland aquaculture. The
water quality parameters of greatest relevance are Ammonia, Nitrite and
Nitrate concentrations, and accordingly, improved designs and
technologies to perform nitrification as well as denitrification, have
been researched intensively. However, research into systems having to do
specifically with biological nitrate reduction by process of
denitrification, in systems used for aquaculture, are lagging behind.
Those designs that have been suggested, though relatively effective, are
large and cumbersome, difficult to maintain, and therefore expensive. The
two major problems characterizing the existing denitrification systems
used nowadays are: i) the addition of the correct amounts of soluble
carbon compounds (such as methanol) to support bacterial growth is
difficult to maintain (due to fluctuations of water flow rate and nitrate
levels) and therefore, might leach and contaminate the system water; and
ii) high levels of oxygen in the biofilter inflow (close to saturation
due to intensive aeration of the ponds) inhibit denitrification and cause
partial aerobic degradation of the organic carbon applied. Thus, these
denitrification systems require larger systems in order to compensate
lose of organic matter in aerobic metabolism.

[0008]It is well known that the existence of inorganic soluble Nitrogen
compounds is one of the by products of the aquaculture industry. In
general, to remedy this, a biological treatment has been implemented.
However, the biological treatment comprises two processes, i.e. a
nitrification process for converting Ammonia to Nitrate, and a
denitrification process for converting Nitrate to Nitrogen gas. This
biological treatment is the source of some difficulties which are due to
the fact that the two different reaction vessels needed (i.e.,
nitrification and denitrification) require different physical conditions.
Moreover, additional difficulties to be resolved in these systems are due
to the negative influence (reduction in growth) the residual
concentrations of dissolved oxygen in system water (following aeration in
the aquaculture ponds where oxygen reaches saturation) has on the
denitrifying bacteria.

[0009]The currently known denitrifying systems require an external source
of carbon. This source is usually chosen to be a simple and cheap soluble
material such as methanol, ethanol or glucose (Sauthier et al.,
Biological denitrification applied to a marine closed aquaculture system,
Water Research, 1998, Vol. 32, pp. 1932-1938). Anason et al (Limited
water exchange production systems for ornamental fish, Aquaculture
Research, 2003, Vol. 34, pp. 937-941) made a rudimentary attempt to see
if building a recirculating system is possible using only the most
minimal of capital investments. This study showed that by providing even
the most basic of biological filters, it becomes possible to decrease the
amount of water needed in order to deal with inorganic nitrogen
accumulation.

[0010]One experiment (Menasveta, P. at al., Design and function of a
closed, recirculating seawater system with denitrification for the
culture of black tiger shrimp broodstock, Aquacultural Engineering, 2001,
Vol. 25, pp. 35-49) was done to evaluate the effectiveness of a
zero-exchange recirculating system on the basis of water quality
parameters. In terms of the denitrifying column, three different
substrates were used. The results of this project showed that by using
this design, most of the checked water quality parameters stayed within
acceptable parameters with the exception of Nitrate. While this study
showed that implementation of such a system is possible, improvements are
still needed. Additionally the system setup employed expensive methods
such as physical oxygen removal of oxygen via gaseous N2, and then
reoxygenation. Furthermore, no attention was paid to the possibility of
methanol/ethanol concentration buildup which can be toxic.

[0011]A different approach towards the same problem was attempted by Shnel
et al., (Design and performance of a zero-discharge tilapia recirculating
system, Aquacultural Engineering, 2002, Vol. 26, pp. 191-203). Solids and
backwash water, captured by the physical filter were diverted to a
sedimentation basin. The denitrification process reduced the Nitrate
content of the basin water, which was thereafter pumped back into the
rearing tanks. This process was unsuccessful as a relatively high Nitrate
concentration was quickly reached whereupon it stabilized.

[0012]An additional attempt to curb the increase of Nitrate was made by
Suzuki et al., (Performance of a closed recirculating system with foam
separation, nitrification, and denitrification units for intensive
culture of eels: towards zero emission, Aquacultural Engineering, 2003,
Vol. 29, pp. 165-182), wherein methanol, which served as the carbon
source, was pumped into the denitrifying biofilter. The results of this
study showed that this type of denitrification system has a high
potential. Incorporating this type of filter and similar carbon sources
could be effective in a zero-discharge recirculating system. The
disadvantages of this system are mostly related to the extremely large
size of the denitrification unit, and the lack of a deoxygenating method.
For this system to be implemented into large scale use, initial capital
investment might in fact be too high for the system to be economically
feasible.

[0013]Though the results seem positive (Suzuki et al., 2003), a question
still remains with the possible adverse effects of the addition of
methanol, ethanol or glucose into marine culture systems. In order to
cope with this problem, an additional carbon source, one that is not
water soluble should be used. Soares et al., (Denitrification of
groundwater: pilot-plant testing of cotton-packed bioreactor and
post-microfiltration, Water Science and Technology, 2000, Vol. 42, pp.
353-359) showed that using cotton wool as a carbon source can also be
effective in coping with Nitrate. Cellulose is the most abundant
renewable resource in the world and constitutes a high proportion of both
agricultural and domestic wastes. Using this design, Soares et al (2000)
showed that achieving almost total denitrification is indeed possible
without using soluble carbon sources. The downside of this study was that
the size of the reactor was considerably large, and frequent clogging
problems occurred due to the compaction of the cotton bed.

[0014]A nitrogen treating method and system for a nitrogen compound is
described in U.S. Pat. No. 6,984,326, which attempts to reduce the size
and cost of the treatment apparatus by a treatment process based on an
electrochemical technique, wherein a cathode reaction region and an anode
reaction region are defined by a cation exchange membrane interposed
between a cathode and an anode.

[0015]A system for the treatment of wastewater is described U.S. Pat. No.
6,979,398, said system includes a conventional septic tank and two
sanitization modules connected in series and automatically controlled by
a controller, wherein the first sanitization module includes a
cylindrical container and a filtering pouch, and wherein said cylindrical
container includes small polymer balls used as a non-clogging media to
attract the bacteria injected in the wastewater.

[0016]Japanese Patent No. 6320182 describes denitrification means for
removing nitrogen from water wherein a number of contact filter media
consisting of the nonwoven fabric coated with an insoluble pyridinium
type resin are attached to a water-permeable container at intervals, said
contact filter media are obtained by forming a string or paper strip on a
porous nonwoven fabric consisting of fibers such as rayon, cotton,
polyethylene, polypropylene, etc., having its surface coated with an
insoluble pyridinium type resin having halogenated pyridinium group in
the molecule.

[0017]The methods described above have not yet provided satisfactory
solutions to the currently available biological water treatment methods.
Therefore there is a need for suitable biological treatment systems and
methods that overcomes the above mentioned problems.

[0018]It is therefore an object of the present invention to provide a
system and method for efficiently removing nitrate and nitrite compounds
in water treatment processes which requires significantly reduced vessel
sizes and which allow reducing costs.

[0019]It is another object of the present invention to provide an
apparatus and method for removing nitrate and nitrite compounds in water
treatment processes which do not release organic residuals from the solid
carbon source.

[0020]It is a further object of the present invention to provide an
apparatus and method for removing nitrate and nitrite compounds in water
treatment processes wherein denitrification inhibition due to dissolved
oxygen is eliminated

[0021]It is yet another object of the present invention to provide an
apparatus and method for removing nitrate and nitrite compounds in water
treatment processes wherein the consumption of the carbon source is
substantially reduced.

[0022]It is yet a further object of the present invention to provide a
simple to maintain apparatus for removing nitrate and nitrite compounds
in water treatment processes, wherein the biofilter media may be easily
replaced.

[0023]It is yet another object of the present invention to provide a
simple to construct and maintain apparatus for removing nitrate and
nitrite compounds in water treatment processes, wherein the biofilter
system may be easily enlarged by additional modules to cope with
increasing flow rates.

[0024]It is yet another object of the present invention to provide a
simple to maintain apparatus for removing oxygen using a degassing unit
prior to the denitrification apparatus thus significantly reducing its
size.

[0025]It is yet another object of the present invention to provide a
simple to maintain apparatus for removing excess CO2 from the
aquaculture system through a degassing unit.

[0026]Other objects and advantages of the invention will become apparent
as the description proceeds.

SUMMARY OF THE INVENTION

[0027]The present invention generally relates to the treatment of nitrate
rich water, particularly aquaculture pond water. The present invention
provides a two-stage treatment process, wherein in the first stage a
degassing chamber is used to remove dissolved oxygen from a stream of
water flowing out of the aquaculture system, and in the second stage the
stream of water obtained from said degassing chamber is flown into a
denitrifying biofilter comprising a biofilter media which functions as a
biological growth media and as a carbon source, wherein said denitrifying
biofilter is capable of biologically reducing both nitrate and nitrite
compounds into nitrogen gas.

[0028]The inventors of the present invention discovered that
denitrification of water can be carried out efficiently utilizing
relatively small (e.g., 45 liter biofilter for a 13 m3 aquaculture
pond) treatment vessels, while minimizing release of organic residuals,
preventing inhibition of denitrification, and simplifying maintenance and
reducing costs.

[0029]In one aspect the present invention is directed to a denitrification
apparatus comprising a degassing chamber adapted to remove dissolved
oxygen from a stream of water flown thereinto, and an anoxic biofiltering
means capable of carrying out denitrification of a stream of water
received from said degassing chamber.

[0030]The degassing chamber may be implemented by a relatively small
(e.g., approximately 5 liters) tank having a water inlet provided in the
upper portion of said tank and a water outlet in the lower portion of
said tank, preferably in its base, wherein said water inlet is connected
to a spray nozzle installed in said tank near said water inlet, and
wherein a degassing apparatus such as a vacuum pump (e.g., venturi vacuum
pump) connected to an upper portion of said tank, preferably to its
ceiling, is used for applying negative pressure conditions (e.g., 0.1-0.3
bars) thereinside.

[0031]The anoxic biofiltering means may be implemented by an elongated
vessel comprising a water inlet and a water outlet provided in opposing
sides thereof such that water streamed therethrough is flown along the
length of said vessel, one or more biofilter medias disposed along the
length of said vessel covering cross-sectional sections thereof such that
water flown thereinside is forced to pass through said biofilter medias,
and a plurality of spacer elements filling sections of said vessel. The
biofilter medias preferably comprise materials (e.g., cotton) capable of
functioning as growth media and as a Carbon source. The spacer elements
are preferably small (e.g., having a diameter of about 5-8 mm) porous
balls or beads.

[0032]A water pump may be used for supplying the stream of water to the
degassing chamber.

[0033]In another aspect the present invention is directed to a method for
denitrifying water, the method comprising: providing a stream of water,
removing dissolved oxygen from said stream of water and thereafter
filtering said stream of water by means of one or more biofilter medias
capable of functioning as growth media and as a Carbon source.
Advantageously, the filtering is carried out in an elongated vessel
having the one or more biofilter medias installed along its length,
wherein the water is flown along the length of said elongated vessel. A
uniform water stream may be obtained in the elongated vessel by means of
a plurality of spacer elements filling portions of said elongated vessel.

[0034]In yet another aspect the present invention is directed to a water
treatment system comprising: a source of water, a degassing chamber
adapted to receive a stream of water from said water source and remove
dissolved oxygen therefrom, an anoxic biofiltering means adapted to
denitrify a stream of water received from said degassing chamber by means
of a biofilter media capable of functioning as a biological growth media
and as a carbon source, an aerobic biofiltering means adapted to receive
water stream from said water source and from said anoxic biofilter and to
provide a nitrified stream (ammonia-free filtrate--following biological
nitrification) to a water filtering means connected thereto. The water
filtering means is preferably a type of particle sand filter aimed at
purifying the water from suspended and colloid residuals for producing
clear water. Advantageously, the anoxic biofiltering means is implemented
by an elongated vessel one or more of the biofilter media disposed along
its length and a plurality of spacers filling sections of said elongated
vessel.

[0035]The water treatment system of the invention may be further used for
removing excess CO2 from the aquaculture system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]The present invention is illustrated by way of example in the
accompanying drawings, in which similar references consistently indicate
similar elements and in which:

[0037]FIG. 1 is a block diagram schematically illustrating a water
treatment system according to a preferred embodiment of the invention;

[0040]FIG. 3B is a perspective view of a preferred embodiment of the
biofilter media;

[0041]FIGS. 4A and 4B are graphs showing nitrate concentrations obtained
with two experimental implementations of the invention; and

[0042]FIG. 5 is a graph showing the results obtained with an
implementation of the invention without the degassing chamber.

[0043]It should be noted that the embodiments exemplified in the Figs. are
not intended to be in scale and are in diagram form to facilitate ease of
understanding and description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0044]The present invention relates to a Nitrogen treatment system and
method for treating water containing inorganic Nitrogen (Nitrate and
nitrite), such as nitrate rich aquaculture water. The Nitrogen treatment
of the present invention incorporates a two-stage approach in carrying
out the denitrification process. In the first stage, dissolved oxygen is
removed from the water by a degassing chamber, thereafter the water is
flown from the degassing chamber into a denitrifying biofilter, wherein
the biofilter media (e.g., cotton-wool), is used as a biological growth
media and as a carbon source, which serves as a primary electron donor.

[0045]FIG. 1 is a block diagram schematically illustrating a water
treatment system 10 according to a preferred embodiment of the invention.
Water treatment system 10 circulates the water in pond 11 through three
treatment subsystems: i) an anoxic denitrification subsystem 10a; ii) an
aerobic nitrification filtration subsystem 12; and iii) a physical
filtration system 13. The three sub-systems may be operated
independently. Alternatively and preferably, the water stream 17s
obtained from anoxic denitrification subsystem 10a is fed into aerobic
nitrification filtration subsystem 12. In a preferred embodiment of the
invention, aerobic nitrification filtration subsystem 12 receives two
water feeds: i) a water stream 11s provided directly from pond 11; and
ii) water stream 17s obtained from aerobic nitrification filtration
subsystem 12. This flow arrangement enables complete removal of nitrites
(which is a more toxic substance of the two, nitrate and nitrite) by a
two fold action: denitrification (reduction of nitrite into nitrogen gas)
in the anoxic biofilter 17 provided in anoxic denitrification subsystem
10a; and nitrification (oxidation of nitrite to nitrate) in aerobic
nitrification filtration subsystem (aerobic biofilter) 12.

[0046]Anoxic denitrification subsystem 10a comprises a vacuum degassing
chamber 16, which receives a stream of pond water 11p from pond 11, and
an anoxic biofilter 17, which receives a stream of water obtained from
vacuum degassing chamber 16 and outputs a water stream 17s supplied to
aerobic nitrification filtration subsystem 1b.

[0047]Aerobic nitrification filtration subsystem 12 comprises an aerobic
trickling biofilter which provides a stream of water (the obtained
filtrate) to physical filtration unit 13, said aerobic nitrification
filtration subsystem 12 receives a stream of pond water 11s (containing
ammonia) and outputs a stream of water 12s which is supplied to said
physical filtration unit 13. A water stream 13t provided by filtration
unit 13 is reintroduced into pond 11, and a portion of this stream 13s is
supplied to protein fractionator 14, which is used for removing organic
matter and fine solids therefrom.

[0048]With reference to FIG. 2, vacuum degassing chamber 16 comprises a
water tank 21 having a water inlet 23 preferably provided in an upper
portion of said water tank 21, a water outlet 24 preferably provided in a
lower portion of said water tank 21, and vacuum pump 22 provided in an
upper portion of said water tank 21, preferably in its ceiling. A water
pump 28 may be used for streaming water from pond 11 into water tank 21,
via water inlet 23. Said water inlet 23 is connected to a spray nozzle 25
assembled inside water tank 21. In this way the water stream (11p)
supplied by water pump 28 is sprinkled inside water tank 21 via spray
nozzle 25 such that dissolved gaseous O2 is effectively stripped
therefrom by means of vacuum pump 22. Additionally, degassing chamber 16
may be used to resolve further problems associated with intensive
aquaculture systems wherein there is accumulation of carbon dioxide gas
in the system water. Namely, the CO2 accumulated in the water can be
stripped simultaneously with the stripped oxygen and thus reduce
quantities of chemicals needed for pH control.

[0049]Pond 11 is typically a man made water reservoir capable of holding
water volumes needed for growth and reproduction of a variety of
aquaculture products. The water in pond 11 may comprise mixtures of
freshwater and seawater (up to 40 g/l) to enable growing of marine and
freshwater organisms (e.g fish, crustacean invertebrates or algae). The
shapes of the tanks and the drainage systems should be specifically
adapted to each production scheme.

[0050]Water tank 21 may be any type of metallic or plastic vessel capable
of maintaining the needed pressure conditions needed for the oxygen
stripping to take place. In a specific embodiment of the invention, water
tank 21 employed is a relatively simple system designed to occupy a
volume of about 10 liters (e.g., for handling a 13 m3 aquaculture
pond), operated with a very low vacuum of about 1 psi. With such
operational parameters oxygen concentrations in the treated water may be
reduced from saturation to zero.

[0051]A special experiment was conducted to assess the efficiency of
CO2 stripping by the experimental system using pond water bubbled
with CO2 to an average CO2 concentration of 1,200 mg/L. In this
experiment it was found that 50% of the CO2 could be stripped under
the mild vacuum conditions applied.

[0052]Vacuum pump 22 may be implemented by any suitable pressure pump
capable of applying negative pressure conditions in water tank 21. In a
specific embodiment of the invention said pressure conditions is in the
range of 100 to 500 mbar, preferably about 100 mbar if oxygen stripping
only is required. Preferably, vacuum pump 22 is a type of venturi vacuum
pump, such as, but not limited to, JD-100M-STAA4 manufactured by Vaccon
(USA). In the specific embodiment of the invention water pump 28 may be
implemented by a small pump capable of providing flow rates in the range
of 10 to 30 liters/h, preferably about 20 liters/h.

[0053]Degassing chamber 16 may be placed above anoxic biofilter 17.

[0054]With reference to FIG. 3A, wherein a cross-longitudinal view of
anoxic biofilter 17 is shown, which comprises an elongated vessel 30
having a water inlet 33 and a water outlet 32, said water inlet 33 and
water outlet 32 are preferably provided in opposing sides of said
elongated vessel 30 in order to obtain liquid flow along its length.
Advantageously, water inlet 33 is centered in the side of elongated
vessel 30 opposing the side wherein water outlet 32 is located. This
configuration of anoxic biofilter 17 provides a side-flow regime thereby
obtaining reduced hydraulic pressure on the biofilter media 36 provided
in elongated vessel 30.

[0055]The biofilter media 36 located inside elongated vessel 30 should fit
into cross sectional portions thereof such that the liquid stream passing
thereinside is forced to pass through said biofilter media 36. The space
between adjacent biofilter media 36 sections inside elongated vessel 30,
and between said biofilter media 36 and the sides of elongated vessel 30,
is filled with beads 35, which are used to increase the surface area of
biofilter 17 and thereby provide a uniform liquid flow along the length
of elongated vessel 30, and for providing support for biofilter media 36
disposed thereinside. This structure of anoxic biofilter 17 increases
biofilter media 36 surface area despite compression, by preventing
pressure drops and enabling simple replacement of the filtering media 36.

[0056]In a preferred embodiment of the invention elongated vessel 30 is a
cylindrical elongated vessel and biofilter media 36 disposed thereinside,
as illustrated in FIG. 3B, is shaped in a form of a disk 36d having a
circumferential projection 36p at the boundaries of one side thereof. In
this way water can continuously flow through elongated vessel 30 without
occurrence of pressure drops and compaction of the biofilter media 36.

[0057]Elongated vessel 30 may be any type of metallic or plastic vessel.
In a specific embodiment of the invention, elongated vessel 30 is a
cylindrical vessel having a volume in the range of 30 to 80 liters,
preferably about 50 liters, having a length generally in the range of 50
to 80 cm, preferably about 65 cm, and a radius generally in the range of
10 to 20 cm, preferably about 15 cm. In such specific embodiment the
width of biofilter media (modules) 36 may be in the range of 10 to 20 cm,
and the number of modules disposed along elongated vessel 30 is
preferably in the range of 5 to 10.

[0058]Biofilter media 36 preferably comprise materials that can serve as a
solid carbon source, such as, but not limited to, raw cotton or straw,
preferably cotton wool. Biofilter media 36 may be encased in a
metallic/plastic net configured in a desirable shape, such as shown in
FIG. 3B, said metallic/plastic may have aperture size in the range of 50
to 100 mm, preferably about 80 mm. In a preferred embodiment of the
invention biofilter media 36 is made entirely from cotton wool, which
serves dually as biological growth media and as carbon source for
denitrification bacteria. Beads 35 are preferably small porous balls
having a diameter generally in the range of 5 to 10 mm, preferably about
8 mm. Beads 35 may be made from plastic. Beads 35 serve as spacers, for
reducing biofilter media 36 overall compressibility, and also serve to
homogenize the liquid flow through elongated vessel 30.

[0059]In a specific embodiment of the invention the flow rate through
anoxic biofilter 17 may generally be in the range of 10 to 30 liter/h,
preferably about 20 liter/h. Anoxic biofilter 17 may be placed directly
under the degassing chamber 16.

[0060]Aerobic trickling biofilter of aerobic nitrification filtration
subsystem 12 may be any type of aerobic trickling biofilter as commonly
used in the aquaculture industry. The physical filtration 13 is
preferably carried out by means of a particulate sand filter, for
example, Astral 750, manufactured by Astarlpool (Spain). Of course, other
conventional filters may be equally employed for the same purpose.
Protein fractionator 14 may be implemented by any suitable fractionator
as commonly used in the aquaculture industry.

Example

[0061]The following non-limiting example presents results obtained in an
experimental setup of the present invention.

[0062]The anoxic biofilter (˜50 liter) was constructed from a PVC
pipe that was filled with commercial cotton wool (such as commercially
available in pharmacies), and plastic beads packed in the manner
illustrated in FIG. 3A. Cotton wool served as the main carbon source for
the denitrifying bacteria as well as its growth medium due to its low
cost, availability, low water solubility, and due to the fact that it
does not breakdown into other organic compounds. In this system, the
beads served primarily as spacers, which help to reduce the overall
compressibility of the cotton. This increased the active zone (zone which
the denitrification takes place) and thereby increased overall
effectiveness. The total amount of beads in the column was approximately
26 liter, and the total cotton wool content was about 1.1 kg. Upstream to
the anoxic biofilter, a small (10 liter) plastic degassing chamber, which
was placed above the cotton-wool-filled column, was used for physically
stripping the water of dissolved gaseous O2 by means of a Venturi
vacuum tube (Vaccon JD-100M-STAA4), thereby eliminating the need for
other degassing techniques such as the bubbling of Nitrogen gas. The
influent pipe of the degassing chamber comprise a spray-like ending
inside degassing chamber, containing a number of small holes; thereby
increasing the surface-area to volume ratio of the water to be
deoxygenated. Using this approach an effective and low maintenance system
was produced which enabled effective denitrification.

[0063]The experimental set-up was situated in a greenhouse at the
Ben-Gurion University of the Negev, Beer-Sheva, Israel. Beer-Sheva is
located inland approximately 60 km from the nearest coastline. Two
artificial shrimp ponds were located inside a dark room (6×12 m)
that occupied half of the greenhouse. Water from the ponds was allowed to
flow out of the dark room into separate water treatment facilities that
occupied the other half of the greenhouse. Each water treatment facility
included: an aerobic biofilter, a pump (UltraFlow, Pentair Pool Products,
USA), a particulate sand filter (Astral 750, Astarlpool, Spain). The
denitrifying biofiltration system and a foam fractionator (Fresh-Skim
200, Sander, Germany) were assembled in parallel to the main water flow.
A small aquarium pump fed the water from the shrimp pond directly to the
denitrifying biofiltration system, and the water stream obtained from its
outlet was flown to the aerobic biofilter (FIG. 3A). The aerobic
biofilter comprised a polyethylene container (˜100 liter) filled
with plastic beads (Aridal Bio-Balls, 860 m2 of surface area and 160
kg per cubic meter, Aridal, Israel). Each pond was filled with 13 m3
synthetic brackish water and was maintained at 29±1° C.
Synthetic brackish water was prepared by raising the salinity of local
tap water to 4 ppt (Atkinson and Bingman, 1997 (Atkinson, M. J., Bingman,
C., 1997. Elemental composition of commercial seasalts. J. Aquaricult.
Aquatic. Sci. 8, 39-43.) with synthetic sea salts (Red Sea Salt, Red Sea,
Israel). Pond biomass density was approximately 590 g/m3, and dry
feed constituted approximately 3.5% of total biomass a day.

[0064]FIGS. 4A-4B show the results obtained with both systems after 115
days, wherein FIG. 4A shows the N-Nitrate concentrations in the first
experimental system and FIG. 4B shows the N-Nitrate concentrations in the
second experimental system. The results of both systems suggest that
maintaining a low nitrate level in system water is possible. Starting
Nitrate levels were very high and a sharp decline was evidenced after
approximately 2 weeks. After the sharp decline, nitrate levels remained
stable at approximately 6-7 mgN/l.

[0065]In comparison, a similar experiment was initially completed with the
intent to understand the importance of the degassing pre-treatment phase.
FIG. 5 shows the results of the preliminary system, without the degassing
chamber. Starting N-Nitrate levels were low initial N-Nitrate
concentrations. However due to a steady increase in N-Nitrate
concentrations due to reduced biofilter efficiency, the system reached a
final steady-state concentration of approximately 60 mg/l as N.

[0066]The following table lists various parameters of the experimental
setup exemplified hereinabove.

[0067]As was described and exemplified hereinabove, the present invention
provides efficient nitrate removal scheme for water treatment processes,
wherein the facilities used for carrying out the denitrification are of
relatively small sizes and employs relatively inexpensive means. Among
the many advantages of the invention, the following are particularly
desirable in aquaculture systems: [0068]1. No release of organic
residuals from the solid carbon source (cotton). [0069]2. No inhibition
of denitrification due to the removal of feed water dissolved oxygen by
the degassing device. [0070]3. Reduced consumption of the carbon source
due to more efficient denitrification (no aerobic consumption of cotton).
[0071]4. Physical filtration of the treated water and preservation of
denitrification bacteria within the biofilter by the cotton-bead media.
[0072]5. Reduction of biofilter volume due to the increased process
efficiency. [0073]6. Simple maintenance (no need to dose a continuous
liquid carbon source). [0074]7. Easy replacement of biofilter media and
of modular expansion.

[0075]It should be appreciated that the denitrification system of the
invention is simple to construct and maintain and that this innovative
biofilter system may be easily enlarged by the addition of bed modules to
cope with increasing flow rates. In addition, the size of the
denitrification system of the invention is significantly reduced,
compared to systems of the prior art, due to the oxygen removal unit
employed. Additional advantages of the invention are in its ability to
remove excess CO2 from the aquaculture system through the degassing
unit in that it may prolong the time periods of using the same body of
water (i.e., water saving) and prevents the release of contaminated water
into the local sewage system.

[0076]It should be noted that the present invention may be employed in
other applications involving anaerobic bio-filters for the treatment of
water and wastewater.

[0077]All of the abovementioned parameters are given by way of example
only, and may be changed in accordance with the differing requirements of
the various embodiments of the present invention. Thus, the
abovementioned parameters should not be construed as limiting the scope
of the present invention in any way. In addition, it is to be appreciated
that the different vessels, tanks, and other members, described
hereinabove may be constructed in different shapes (e.g. having oval,
square etc. form in plan view) and sizes differing from those exemplified
in the preceding description.

[0078]The above examples and description have of course been provided only
for the purpose of illustration, and are not intended to limit the
invention in any way. As will be appreciated by the skilled person, the
invention can be carried out in a great variety of ways, employing more
than one technique from those described above, all without exceeding the
scope of the invention.